Several recent areas of interest in robotics require the direct physical interaction of an operator with manipulators, haptic displays, or assistive devices. Haptic displays, which are essentially robots designed for direct, physical interaction with human operators, have a great variety of applications. These applications can range from teleoperation, to virtual reality, to robotic surgery.
Haptic displays can be used to implement programmable constraint where, for example, haptic virtual fixtures (i.e. hard walls which constrain motion to useful directions) can improve performance in teleoperation tasks such as a remote peg-in-hole insertion task. Further, in computer-assisted surgery, a surgeon may move a surgical tool cooperatively with a robot, with the robot enforcing certain constraints (e.g. "cut only this surface") while the surgeon is free to maneuver the tool within the allowed surface. In virtual reality applications, a user may interact with and "feel" objects which exist only in digital representation, by using a haptic display to probe those virtual objects and perceive the resulting reaction forces. In vehicle final assembly, workers may cooperatively control the motion of large, heavy vehicle components with an assistive device, where the assistive device controls some aspects or bounds on its motion, while the worker controls others.
The aforementioned examples have in common the exchange of force and motion between a human and a multi-degrees of freedom robot. For example, the examples involve the commonalities of constraining the motion of a human operator and the human operator supplying the source of energy for carrying out the particular task. Different requirements apply to robots such as these, that interact energetically with an operator, than apply to industrial robots in a typical humans-excluded environment. First, the development of perceptually smooth force-following, in which the robot is guided by the user, has been found to be quite difficult and to require much higher servo rates than are needed for position control alone. Second, passivity and stability of the robotic system is required when it is considered that a robot with a payload of only a few pounds can quickly develop lethal kinetic energy and when unpredictable human impedance is possible during operation.
Conventional passive haptic constraint devices typically have been provided with as many degrees of freedom as the resulting device is expected to exhibit and its joints are driven by motors to this end. If the motors are inactive, the user can move the device freely. When it is desired to reduce the number of degrees of freedom (in order to implement a software constraint surface), the motors are activated to couple the motion of the joints so that the user's ability to move the device is constrained.
However, when using conventional approaches to haptic constraint devices where servo control is used to reduce the degrees of freedom of the robot to those consistent with the programmed constraint, passivity and constraint have been antagonistic goals. In particular, to implement an effective constraint, a servocontroller requires high gains which are incompatible with robotic passivity and stability. Such disadvantages are inherent to the servo controlled approach to haptic displays.
Passive constraint devices have appeal for operating in continuous time and in a perceptually smooth manner in response to forces applied to it by a user. The passive constraint device lacks external energy sources and its kinetic energy is limited to that provided by the user, therefore a hardware or software failure cannot produce high velocities that might injure the user and/or the robot.
One passive haptic constraint device (M. Russo and A. Tadros, Controlling Dissipative Magnetic Particle Brakes in Force Reflective Devices, ASME Winter Annual Meeting, Anaheim, Calif., pp. 63-70, 1992) employs controllable brakes rather than (or in addition to) servoed actuators at the joints of a device. Brakes can implement very hard constraints and are completely passive. However, a braked passive constraint device suffers from a serious disadvantage in that it is limited to the types of virtual surfaces which it can implement. For example, considering a passive constraint device having brakes that activate in the x-axis and y-axis directions with respect to a two-axis Cartesian coordinate, implementation of a 45 degree wall relative to the x and y axes would require a series of step motions that can be perceived by the user and that can result in both the x-axis and y-axis brakes both being activated in a manner that motion becomes impossible along the 45 degree wall.